The present study deals with the transport activities at the serosal and luminal surfaces of the isolated frog gastric mucosa. In particular uptake of C1 ion at these surfaces is under investigation. Active transport of C1 is responsible for electrical activity in this preparation, and uptake of C1 at the serosal surface may be the limiting factor in the capacity of the stimulated mucosa to produce acid. In addition we plan to use the drug ouabain in experiments to determine more precisely the part played by the Na ion K activated ATPase in the maintenance of cellular levels of Na and K and acid secretion.